Animal Companions
Animal Companions in CoA have been altered from the traditional Animal Companion system NwN uses. The Animal Companion Summon button will no longer do anything, instead the system uses a series of dot commands that you can use via chat (or set in your quickbar using macros).
These commands allow you to tame many animals/vermin you might find in the wilderness, provided their HD is equal or lower than your druid's level and you succeed an Animal Empathy check.
Disclaimer: While you can acquire companions in this way, if they die in combat they are gone for good and you'll need to find and tame a new companion.
Other disclaimer: Not every animal/vermin found in coa is tamable, for balance reasons. Also if you have less than 10 charisma your companion will receive penalties to its stats.
-- SPIFFY NOTE: Some animal companions are nerfed. They will not scale as described below. Others will scale better. It depends on what you find.
The dot commands are as follows
".compantame" Will tame the animal or vermin nearest to your druid, if the creature is non hostile the DC to tame it will be lower, you can use Animal Empathy to make animals non hostile and increase your chances of taming them.
".compansum" Will summon and unsummon your animal companion, keep in mind that you can not immediately resummon a companion after unsummoning it, or after it's been dismissed using the dismissal spell. You'll have to wait a certain time before being able to resummon it (even longer if it's been dismissed). The exception is the first time you unsummon a companion after taming it (allowing you to rename it).
".companname" Will set a new name for your companion, the next time you resummon them, their name will be whatever you set it to.
Additional Animal Companion Info
Taming Levels
Druids can tame companions equal or lower than their total druid levels.
Rangers can tame companions equal or lower than their ranger levels -2 levels (since they obtain the ability to tame at level 3).
Ranger and druid levels will combine when determining your ability to tame companions (the ranger's -2 level penalty still applies to ranger levels)
For every level a druid exceeds that of their Animal Companions HD, the companion will receive a +1 bonus to Str, Con, and Wisdom.
For every 2 levels a druid exceeds that of their Animal Companions HD, the companion will receive a +1 bonus to Dex and dodge AC
Charisma Bonuses
Charisma will give benefits to an Animal Companions stats if it's higher than 10, up to a maximum of 14 charisma.
11 Charisma: Gives the companion +2 Wis
12 Charisma: Gives the Companion +1 Str, +1 Dex, +1 Con, +1 Wis, +1 Dodge AC
13 Charisma: Gives the Companion +2 Str
14 Charisma: Gives the Companion +1 Str, +1 Dex, +3 Con, +1 Wis, +1 Dodge AC

Familiar Changes:
On Arabel, familiars were changed into more natural companions, mostly animals. These changes affect both wizards and sorcerers alike. The list of familiars changed can be found at the bottom of this topic.
Cosmetics Change:
Familiars were given reskins to look like animals instead of monster pets. Though they will still appear as their respective monster in the familiar selection menu, they end up turning into those animals when summoned, below is the list of changes.
Eyeball replaced by Rat
Fairy Dragon replaced by Spider
Fire Mephit replaced by Snake
Hell Hound replaced by Dog
Ice Mephit replaced by Beetle
Imp replaced by Parrot
Panther replaced by Cat
Pixie replaced by Chicken
Psudodragon replaced by Lizard
Raven remains Raven
Bat remains Bat
Mechanical Change
Familiars are greatly reduced in power, on Arabel a familiar can not have more than half the wizards total HP, they will be damaged down accordingly if they exceed that much.
Familiars have had some stats removed such as damage reduction, regeneration, and spell like abilities.
They have penalties to skills, like Disable Device and Open Lock.
These changes were done because familiars are not tanks, mages, or rogues. They are companions and friends meant for roleplay.
Familiars will however grant a minor passive bonus to the wizard if nearby, depending on which familiar you choose.

The Uniter
The legend of the one who will unite all the tribes into an unstoppable army to sweep all over Faerun is understandably a compelling one for most barbarian tribes, who shed more blood warring against each other than any outsider. While only a handful of people in history have ever actually managed to unite the tribes, many clan believe they have the next chosen one in their midst after consulting their wise women's augurs (who more often than not, simply tell the chieftains what they wish to hear), and train a child in the ways of leadership, promising them a place in history.
Passive Bonuses:
+5 bluff, persuade, Intimidate
+3 taunt,
+1 Dodge AC
Rage Bonuses:
-2 AC
+2 Will
Add Charisma modifer to STR/CON to all party members (+4 cap)

Lawful Good - Paladin of Honour
Code of Conduct: A paladin must be of lawful good alignment and loses all class abilities if she ever willingly commits an evil act.
Additionally, a paladin’s code requires that she respect legitimate authority, act with honor (not lying, not cheating, not using poison, and so forth), help those in need (provided they do not use the help for evil or chaotic ends), and punish those who harm or threaten innocents.
Associates: While she may adventure with characters of any good or neutral alignment, a paladin will never knowingly associate with evil characters, nor will she continue an association with someone who consistently offends her moral code. A paladin may accept only henchmen, followers, or cohorts who are lawful good.
Lay on Hands: As normal.
Aura of Courage: As normal.
Remove Disease: As normal.
Summon Mount
A paladin of Honour can call a celestial squire, in the form of a Hound archon to their side using their summon mount ability.
A celestial squire is a companion who unswervingly serves a paladin in combat.
The Hound Archon is able to heal, and remove lesser ability drains.